1. Field of the Invention
The present invention relates to a method and apparatus for inserting a communication wire through a pipeline, in which the communication wire, such as an optical fiber cable or the like, coated with an insulating material is inserted through the pipeline by air flow, and particularly relates to the method in which the resistance of propelling the communication wire is reduced.
2. Description of the Related Art
In a method of inserting a communication wire material through a laid pipeline by air flow while blowing compressed air therein, it is important to reduce the friction coefficient between the communication wire and the inner wall of a pipeline. For example, in an optical fiber cable disclosed in Japanese Patent Unexamined Publication No. Hei-2-50111, the friction coefficient is reduced by adding a solid lubricant to an outermost layer of a coating layer which is a contacting surface with the inner wall of a pipeline. In addition, a method of laying an optical fiber cable in which blowing powder talc as lubricant is disclosed in Japanese Patent Examined Publication No. Hei-2-22921.
In general, a pipeline through which a communication wire is inserted is made of insulating resin. The coating layer of the communication wire is made of an insulating resin. With respect to an optical fiber cable, for example, the cable is usually coated with foamed polyethylene and it is pressed and inserted into a polyethylene pipeline. As for the gas for propelling the cable, dried air or nitrogen is usually used in order to avoid increasing the frictional resistance due to moisture.
There is therefore a tendency to cause static electricity by friction when a communication wire is pressed and inserted into a pipeline by air flow. If static electricity is generated, resistance is produced by the electrostatic force so that frictional resistance increases. In the above-mentioned methods, there is indeed an effort to reduce the friction coefficient, but there is no effort to reduce static electricity. Although a method of coating the surface of a coating layer of a communication wire with an antistatic material such as diethanolamine or the like has been reported, experiments made by the present inventor and others have proven that the antistatic material is shaved from the communication wire by friction against the pipeline. The antistatic effect is reduced if the insertion distance is increased, that is, the greater the insertion distance, the lesser the antistatic effect of the antistatic material.
The following reasons are thought to account for the above-given results.
1. Since such an antistatic material has the function of discharging charged static electricity by absorbing moisture to form a semiconductor film on the material surface, the conductivity thereof comes to be lost if the insertion distance is increased so that the time of sending dried air is prolonged.
2. The antistatic material layer is shaved from the communication wire so that the effect of preventing charging is reduced.
3. If the insertion distance of the wire is increased, the frequency of friction between the pipeline and the communication wire is increased, and the possibility of generating static electricity is increased.
Thus, in a method of inserting a communication wire through the pipeline by air flow, the attraction between a pipeline and the communication wire is increased by the charging of static electricity at the time of propelling by the velocity of the propelling and the frictional resistance is increased by increasing the insertion length of the communication wire.